1. Field of the Invention
The present invention relates to a method of manufacturing an optical device, and, an optical device manufactured by the method.
2. Description of the Background Art
The optical subscriber line system implementing optical transmissions from subscribers to a central office (upstream transmission) and from the central office to subscribers (downstream transmission) over a single optical fiber line may use light beams having wavelengths different from each other between the upstream and downstream transmissions. The current mainstream of the subscriber line system is the GE-PON (Gigabit Ethernet (trademark)-Passive Optical Network), which bidirectionally implements fast communication of 1 Gbps or above. In recent years, investigations have been made on the WDM (Wavelength Division Multiplex)-PON with a higher level of multiplicity in communication wavelength, as a next-generation technology replacing the GE-PON. The WDM-PON can theoretically achieve a communication rate exceeding 10 Gbps in opposite directions.
In a WDM-PON, an optical fiber line for use in communication has its opposite ends provided with a central office-side line terminator, i.e. optical line terminal (OLT), and a subscriber-side line terminator, i.e. optical network unit (ONU), respectively, at the central office and the subscriber set. Those terminators have an optical chip having optical devices integrated thereon, such as light-emitting device, photo-sensitive device and wavelength multiplexer/demultiplexer.
In recent years, a Si optical waveguide comprised of a silicon (Si) core and a silicon oxide (SiO2) cladding has been used for coupling optical devices on the optical chip. The Si optical waveguide, of which the core has its refractive index very much higher than the refractive index of the cladding, shows a strong effect of light confinement. Accordingly, by using the Si optical waveguide, it now becomes possible to form a curved optical waveguide which can bend light with a short radius of curvature of 1 μm or around. Another advantage is that the Si optical waveguide may be manufactured by process technologies for Si electronic devices, capable of achieving an extremely fine cross sectional structure of sub-micron level. Accordingly, by using the Si optical waveguide, the optical chip may be shrunk down to a level equivalent to the Si electronic devices. The Si optical waveguide, therefore, attracts attention as a solution of combining optics with electronics on a single on-chip.
Now, the cross sectional dimension of the Si optical waveguide equals to a fraction of that of external light-emitting device, photo-sensitive device or optical fiber, so that a spot-size converter is necessary in order to optically couple the wavelength to those devices.
Various types of spot-size converters have been proposed. For example, there is known a solution of gradating impurity concentration of the core depending on the distance from the input/output end, so as to decrease stepwise the refractive index of the core of the optical waveguide towards the input/output end, see Japanese patent laid-open publication No. 2004-258610 and U.S. Pat. No. 7,099,540 to Shimoda, for example. The solution is, however, only applicable to quartz-based optical waveguides having refractive index tunable to a desired extent through addition of impurity, and is hardly applicable to the Si optical waveguide which is not tunable in refractive index of the core to a desired extent by addition of impurity.
There is also known a solution of thinning either the width or thickness of the core of an optical waveguide in a tapered manner towards the input/output end, see U.S. Pat. No. 6,937,797 to Mizuno, et al., Japanese patent laid-open publication Nos. 2000-235128 and 2011-123094 for the width thinning, and see Japanese patent laid-open publication Nos. 15435/1997 and 2005-326876 for the thickness thinning, for example. With those solutions, however, a large difference appears between the width and thickness of the core of the input/output end, so that polarization dependence would occur in optical coupling with any of external optical devices.
There is therefore provided a solution of tapering both the width and thickness of the core of an optical waveguide towards the input/output end, aiming at suppressing the polarization dependence, see Japanese patent laid-open publication No. 2010-230982 and U.S. Pat. No. 8,126,301 to Ishizaka, for example. Both patent documents employ highly technical processes including photolithography in order to obtain a tapered form of the optical waveguide, reduced both in width and thickness.
In Japanese patent laid-open publication No. 2010-230982, use is made of an etching product which is generated in the process of dry etching of photoresist. The product has a tendency of being deposited on the surface to be etched composed of Si or the like so as to decelerate the etching. In the Japanese patent publication, making use of that tendency, the optical waveguide is tapered in the thickness-wise direction. More specifically, around the constant-thickness of Si tapered structure which is formed so as to be narrowed towards the input/output end, formed is a resist pattern by which the amount of deposition of the product may be reduced more extensively towards the end of taper. When the structure is dry-etched, the etch depth increases towards the end of taper where the amount of deposition of product becomes more scarce. As a consequence, a spot-size converter having its width and thickness tapered towards the input/output end at the end portion may be obtained.
In U.S. patent to Ishizuka, using an SOI (Si on Insulator) wafer, a spot-size converter having a Si optical waveguide is fabricated which is tapered in the width and the thickness. In more detail, over an SOI layer composed of single-crystalline Si, a SiO2 film is formed. Over the surface of the SiO2 film, a SiN film having a tapered shape in planar view is formed as a mask for suppressing oxidation of the SOI layer. The structure is then subjected to the LOCOS (LOCal Oxidation of Silicon) process. In a portion having a wide mask composed of the SiN film formed thereon, the SOI layer remains unoxidized, and thereby a Si optical waveguide with a large width and a large thickness is formed. Meanwhile in a portion with a narrow SiN film, a Si optical waveguide having a small width and a small thickness is formed corresponding to the mask width. In short, a spot-size converter tapered in the width and thickness is formed right under the SiN film.
Although not relevant to solutions regarding Si optical waveguides, as a solution of enhancing a coupling efficiency of light beam while suppressing variation in dimensional accuracy of the spot-size, there has been proposed a solution of forming the optical waveguide composed of a compound semiconductor by selective epitaxial growth, see Japanese patent laid-open publication No. 114767/1993, for example. According to this solution, over a single-crystalline InP substrate which serves as a cladding, a core composed of InGaAsP, InAlAs or the like is formed by selective epitaxial growth. In that process, by using a selective growth mask composed of SiN or the like for causing an opening to be gradated in the width of opening in a tapered manner, the resultant core will have its transverse cross section shaped into an isosceles triangle with its two equal sides configured by the (111) planes.
The solutions disclosed in Japanese patent laid-open publication No. 2010-230982 and U.S. patent to Ishizuka need, however, complicated manufacturing steps, suffering from dimensional variation in the spot-size converter even under slight changes in manufacturing conditions, and from destabilized coupling efficiency as a consequence.
It has also been understood that the solution disclosed in JP patent laid-open publication No. 114767/1993, based on selective epitaxial growth, is hardly applicable to a Si optical waveguide which uses an amorphous base typically composed of SiO2.
Having exemplified the spot-size converter, the same problems in the dimensional accuracy have been known in other types of devices using a Si optical waveguide, such as grating coupler for use in deflecting the direction of light propagation, and polarization converter for use in shifting polarization planes.